IGNEOUS ROCKS IN YELLOWSTONE PARK. 193 
traversed by sheets of porphyrite, which have been intruded 
from a centre of eruption southwest of the mountain. 
A subsequent synclinal break along what is now the east 
side of the mountain, permitted the further eruption of 
igneous material, which penetrated the nearly vertical 
fissures caused by this fracture. These vertical dikes 
traverse the strata and intercalated sheets in the main body 
of the mountain, but are nearly parallel to them in that 
portion of the mass east of the synclinal axis where the beds 
have been turned up on end. The dikes radiate from 
about the middle of the northeast spur of the mountain 
through an arc of 45° from south to southwest, and are hot 
more than a mile and a half long. At the centre from 
which they radiate is a broad stock of intrusive rocks 1,500 
feet wide, which lies in the axis of the synclinal. From it 
branch several apophyses, and around it the sedimentary 
rocks have been highly metamorphosed. 
The eruptive magmas which found their way between the 
ruptured strata and solidified as intrusive rocks in the form 
of laccolites, sheets, dikes and stocks, also reached the sur¬ 
face of the earth in places and cooled as effusive bodies. 
The extravasated rocks were probably erupted from a num¬ 
ber of different vents, whose position was governed by the 
nature and extent of the fissures in the sedimentary rocks. 
They poured out as flows of lava and were from time to 
time blown to pieces and thrown into great accumulations 
of breccia. These in turn were intersected by dikes and 
stocks of subsequently erupted magmas, that solidified as 
intrusive bodies within the effusive rocks. 
This is the condition of things at Sepulchre Mountain, 
which is directly east of Electric Peak across the valley of 
Reese Creek. The mass of this mountain is made up of 
volcanic breccias and lava-flows and tuffs, which rest upon 
Cretaceous strata and exhibit little or no evidence of bedding. 
The western portion of the mass at the base of the mountain 
is filled with dikes and larger intruded bodies of igneous 
rocks, which also form the low ridges and hills between 
